In a communication system such as a direct sequence spread-spectrum code division multiplex system (DS-CDMA), the uplink signal (the signal from a subscriber to a base-station) comprises a multiplicity of overlapping coded signals from individual subscribers. Each of these signals occupies the same radio frequency (RF) carrier bandwidth simultaneously and are distinguishable only by their specific encoding. The uplink signal is received at a base-station receiver as a composite received signal.
In conventional DS-CDMA systems, the base-station receiver decodes each subscriber separately by applying each respective subscribers'unique spreading code to the composite received signal. Each individual subscriber's signal is thereby "despread" from the composite received signal. Due to the nature of the family of codes utilized, the other subscriber's signals remain in a bandwidth-spread form until they are despread with their particular code. This allows the decoding of user data bits for a particular subscriber.
Traditional DS-CDMA systems endeavor to drive the uplink received signal strength received from different subscribers to the same level. This is done through the use of subscriber power control techniques. In IS-95, the subscribers are constantly given instructions from the base station on how to adjust their power level based on measurements made at the base. If the received subscriber power level falls below a predetermined target, the subscriber is told to increase its power. If the received subscriber signal raises above the target, the subscriber is told to decrease its power. This power control process is intended to optimize system capacity by keeping each subscriber's power at the minimum necessary to achieve a particular quality level. This is because, after despreading, a portion of received power from all other subscribers appears as additional noise (interference) to the despread signal of the desired subscriber. Optimum system performance is achieved when this interference is kept to the least amount possible.
An alternative technique adjusts the power level of subscriber units such that the decoded frame erasure rate (or alternative quality metric) for each subscriber unit, as measured by the base-station, is the same. However, in either technique, the natural and unavoidable variability in received signal strength can lead to capacity degradation. For example, a 3 dB difference between two subscribers can lead to a 1/3 loss of capacity since one of the subscribers will look like twice as much interference (as the other) to the system. The net effect is that the system receives the equivalent power of three subscribers when only two are present, resulting in the 1/3 loss of capacity.
Prior art techniques of interference cancellation are known which act to reduce the interference. This permits an increase in the sensitivity and or capacity of the multi-user system. The most common technique is to synthesize a replica of a particular subscriber's received signal, after it has been properly decoded, and utilize the synthesized replica to cancel interference (by subtraction) in a stored version of the received signal.
In systems which implement interference cancellation, the natural and unavoidable variability in received signal strengths can be exploited by decoding the stronger signals, and cancelling them first. This in fact forms the basis of ordering of decoding for the iterative process in systems which implement interference cancellation. However, the natural variability mentioned above results in only a slight increase in capacity because it is so small and uncontrolled. Furthermore, in pedestrian applications, fading typical of land mobile radio channels becomes very slow, resulting in even less variability than typical land mobile radio channels. In either case, if each of the plurality of signals transmitted by subscribers is at approximately the same power level, the performance of the interference canceller is greatly degraded. It is also necessary to track this natural variability in a continuous fashion in order to know the sequence for subscriber signal decoding.
Therefore, a need exists for a method and apparatus to improve the performance of systems which employ interference cancellation.